Optical devices, such as semiconductor laser transceiver units and light emitting diodes, often have an optical port for transmitting optical energy into fiber optic structures, for example as part of an optical communications system or an optical scanning system. The optical fiber has at its end a connector for enabling the fiber to be connected and disconnected from the port.
Semiconductor laser and light emitting diode transmitter units need to be eye-safe when the connector is not connected to the port. Many semiconductor optical emitters, particularly those of optical communications links, operate at near-infrared wavelengths, e.g., 1.3 μm and 1.5 μm, which present added risk because such wavelengths are invisible. Applicable eye-safety standards for infra-red laser diode transmitter units are the US Standard CDRH Class 1 and the European Standard IEC 825.
Current safety guidelines require the output power density from an optical port of an optical transmitter unit to be limited to a level which is eye-safe when no fiber optic is connected to the port. Optical coupling efficiencies from a laser semiconductor diode into an optical fiber are typically quite low, for example of the order of about 1% to 25%. Even if the amount of optical radiation transmitted by the fiber is eye-safe, the total amount of optical radiation emitted by the semiconductor laser or light emitting diode may far exceed the limit of eye-safety. It is therefore necessary either to block unwanted optical energy within the port, or to defocus stray optical energy emitted by the port when no optical source is connected to the port.
One attempted solution to this problem is disclosed in U.S. Pat. No. 5,315,680, which describes an optical port having a short length of optical fiber, called a “fiber stub.” The stub is held securely in alignment with a laser diode concealed within an optical transmitter unit. Collimating optics focus the laser optical energy into a single-mode core of the fiber. The fiber stub is typically 5 mm to 6 mm long. Optical energy which is not coupled into the core enters the fiber optic cladding and is dissipated by multiple reflections and scattering within the core and the exterior surface of the cladding. Any laser radiation that exits the cladding is not collimated, and is essentially “defocused” to greatly reduce the inherent brightness of such stray radiation.
In recent years there has been an increasing demand for fiber optic communication links having a bandwidth in excess of 1 GHz, for example up to 10 GHz. One way a laser semiconductor diode can operate at higher data rates is to drive the laser at a higher power. It is possible to reduce the amount of optical power launched into a core of a fiber optic structure by defocusing a laser beam focused on an input end face of the fiber stub. Defocusing is achieved by axially offsetting the laser beam waist with respect to the input end face of the fiber optic core. Such an arrangement can also be used to reduce the amount of optical power in the core depending on product specifications and the requirements of various applications. Because the core diameter is much smaller than the cladding diameter more defocused optical energy is incident on the cladding. Thus, there is still more total laser power incident on both the core and the cladding of the fiber stub, to the point where optical energy propagating through the fiber core and/or stub is not eye-safe.
Another problem with using the defocus technique is that the amount of laser power incident on and propagating in the core becomes more sensitive to changes in the relative orientation along the light transmission direction of the fiber stub, the laser and any intervening collimating optics. Such orientations can change because of (1) thermal expansion of components forming the optical transmitter unit, and/or (2) ageing-induced creep of the materials and adhesives used in the construction of the unit.
One way to reduce the laser power propagating from the end of the fiber stub remote from the input end face is to increase the length of the stub. Increasing the stub length increases scattering and absorption over the length of the stub. Cladding modes within a length of optical fiber between about 100 mm and 200 mm long are substantially dissipated. Increasing the stub length undesirably increases the size of the optical transmitter module.
Another attempted solution is to incorporate, at the end of the stub, an aperture, e.g., an absorbing ring around the outside of the fiber core. The aperture, however, must be closely aligned with the core, having a diameter of the order of about 10 μm. This results in additional process steps, which add cost and complexity to the optical transmitter unit.
Commonly assigned Healy (U.S. Pat. No. 6,804,436) discloses an eye-safe optical transmitter unit comprising a laser semiconductor diode for emitting optical radiation. An optical fiber stub having a fiber core carries the optical radiation and is surrounded by a cladding. The core is an index-guided core.
The optical fiber stub is disposed in a ferrule. The ferrule is rotatable in a manner which alters the orientation angle of the entrance face with respect to the beam axis, thereby affecting the efficiency of coupling of the optical radiation into the fiber core and cladding.
Focusing optics focus the optical radiation from the laser semiconductor diode on an entrance face of the fiber stub. The focusing optics focuses the optical radiation along the beam axis to a focus spot on the entrance face of the fiber stub. The entrance face is tilted at a particular orientation with respect to the beam axis.
The focusing optics focus the optical radiation along the focus axis to a focus spot on the entrance face of the fiber stub to increase the coupling efficiency of optical radiation from the radiation source to the entrance face of the fiber core and to decrease the coupling efficiency of optical radiation to the input face of the surrounding cladding. The coupling efficiency into the core is a maximum at a particular orientation of the entrance face with respect to the focus axis when the focus spot is on the entrance face. The entrance face is not oriented at the particular orientation, but is angled and/or rotated away from the particular orientation to reduce the coupling efficiency of the optical radiation from the source to the fiber core and fiber cladding. In one embodiment a polarizer is fixedly attached to the entrance face of the fiber stub at the time the fiber stub is fixedly positioned relative to the optical source.
While the apparatus disclosed in the Healy patent functions admirably for certain circumstances, it is somewhat complex, difficult to assemble, expensive and not applicable to certain situations.
The prior art Healy arrangement has problems if the distribution of light on the input end face of the fiber must be controlled for reasons other than power. For example, if the optical source launches an optical beam into a multi-mode fiber to provide excitation of specific mode groups, the excited modes depend on factors such as beam waist of the source, beam power and beam spatial distribution (i.e., one or more of spot size, spot shape, spot numerical aperture and spot location) on the input end face of the fiber and the alignment of the fiber with respect to the beam axis. Adjusting the alignment of the fiber in all three directions, i.e., in the X, Y and Z directions (where X and Y are axes at right angles to each other in a plane at right angles to the beam axis and Z is the distance between the laser source and the fiber input end face along the beam axis) with respect to the optical source frequently has an undesirable effect on the excited modes in the fiber optic structure. Under certain conditions, it is necessary to launch the optical beam with the fiber at the beam waist. Hence, under these conditions, the prior art alignment technique is sometimes undesirable.
An object of the present invention is to provide a new and improved optical transmitter with a semiconductor optical source and a fiber optic structure and a method of making same, wherein several parameters of the optical radiation incident on end face of the optical fiber element are optically controlled; among the parameters are power and spot spatial distribution.